Recording and reproducing of sound waves



RECORDING AND REPRODUCING OF SOUND WAVES Original Filadl Dec. l?, 1919 2 Shams-Sheet l ATTRNEY L. ESPINSGHIIED RECORDING AND REPRODUCING OF SOUND WAVES 2 Sheets-Sheet Original F'iled Dec. 17

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'llhe invention relates to a method ot and means tor improving the quality and volume ot' reproduction ot sound waves. i

'.lhe object ot the invention is to provide means tor improving the clarity, naturalness and lidelity ot sounds as reproduced trom phonographic devices. 'Another object is the improvement ot the timbre or tone quality vot musical eproduction and a turther object is to enabe the timbre and volume ot the reproduced music to be controlled atthe will ot'. the player or operator.

'lhe tone quality deficiency otx phonographic devices-is caused by distortion ot the sound waves trom their true torms, this distortion occurring during both the recording lili till

.and reproducing processes. @ne important type ot distortion is what may be termed amplitude or volume distortion, arising trom the tact that the instantaneousvalues ot amplitude which a wave' successively assumes are not recorded or reproduced in strict, proportionality.

'llhis amplitude non-proportionality type ot distortion arises wherever the reaction ot the medium in which the translation takes p place, such as the material ot a record, is oth er than a straight-line tunction ot the impressed torce. 'lhe desired condition tor which no distortion ot this type occurs is that thc amplitude ot displacement, tor example, ot the record imprint be directly proportional to the magnitude ot the impressed torce. ,"lFhiscondition is analogous to that obtaining in an electric circuit ot' constant im pedance wherein the current is directly pro portional to the eleotromotive torce. llt the impedance varies with change in current, that is it the current is not a straight line tunction of voltage, then the lcurrent Wave shape is distorted as compared with the voltage wave shape. @similarly in a honograph, if tor instance the resistance ot the record to the impressed scribing torce increases with increased displacement ot the scribing needle into the record, then the shape ot the recorded wave will differ trom and be distorted as co1npared with that yot the impressed torce or original sound wave. Distortion ot this type is more tully explained below in conjunction with the means provided in accordance. with the present invention for correctinor the same.

Another type ot distortion is that due to non-uniformity ot reproduction in respect to trequencies, waves or wave components ot diderent treqluencies being reproduced in disu proportionate amplitudes. 'llhis may be called'resonant distortion since it is caused largelyk bythe natural periods ot the diaphra devices, air chambers,Y horns, etc., ot the recording and reproducing instruments.

' tion, are the causes ot the imperfect reproduction so readily discernible in all ordinary phonograplis, For example, the non-linearity type ot distortion iirst mentioned is particularly responsible tor the disagreeable ettect so etten observed in orchestral pieces where the instruments do not sound individually clear but are rather inter-mixed` or lill dll

,'.lhe distortion, or change in wave shape ot tllll 'litt blurred into a conglomerate. 'lhe resonant or second ty e ot distortion is' discernible as a loss ot tim re richness, and by an over emphasiaing ot certain tones, which, together with the other distortion, produces a monotoynous artihcial characterto the selections. ln

general, distortion ot the iirst type tends to make the wave torni more complex by adding artificially derived trequencies while distortion /ot the second type tends to smooth out the waves and to suppress the component trequencies ot waves originally complex. lUntortunately'these phenomena are not mutually correcting and each has to be treated individually.

The present application is directed at overcoming primarily the iirst type of distortion and secondarily the second type in combination therewith. 'lhe means employed are electrical. l he methods involved, however, are not limited to these means and other means tor applying them will become evident to those skilled in the art.

ln the iigures Figure l delineates in curve torm the manner in which the amplitude type oit distortion occurs.

Figure 2 shows in similar :torni the action involved in correcting such distortion by a counter distortion.

lligure 8 illustrates apparatus means tor ltltll ltllil carrying out this method ofdis'tortion correction d uring the reproduction of a record in a phonograph.

YFigure 4 illustrates thea plication of the correcting process during t 1e taklng of an original record.

Means for correcting simultaneously for both types of distortion noted above are disclosed in Figure 5.

Figure 6 illustrates an alternative arrangement for such combined Correction.

Figure 7- indicates the use for distortion correction of another portion of the characteristic curve of the thermionic amplifier.

Figure 8 represents diagrammatically means whereby the timbre or tone quality as well as the volume of music may be controlled for purposes oi giving personal expression during the rendering ot' a selection.

In Figure 1, curve 1 delineates'the variation in the displacement or amplitude of say the imprint, (plotted as ordinates), with variation in the impressed force (plotted as abscissa) The curve shows that the displacement does not increase directly proportional to but falls olf for each added increment of the impressed force. This is what usually occurs in the making of a phonograph record of the mechanical t pe. The scribing needle as it cuts or pushes into the record encounters increasing] more resistance so that its displacement l'necomes rapidly disproportional to the impressed force. That is, the Wave as recorded is disproportional to and therefore distorted in comparison with the original sound wave. This same eiect is usually present also in the carbon granule microphone type of translating device which sometimes is employed in translating between the air waves of sound and the waves of mechanical motion involved in the recording or reproducing action.

The effect of this non-linear characteristic of the phonograph in distorting and changing the ti bre of a musical note is illustrated in Fig. 1. Iere curve 2 represents a pure tone of sinusoidal wave form impressed upon the phonograph system having the force-displacement characteristic of curve 1. The action of characteristic 1 in translating between what may be termed the input wave 2 and the output wave3 will be apparent upon considerino Fig. 1. Assume, as a concrete case, that the characteristic curve 1 applies to the eect obtained in a phonographic records` er, and in particular to what goes on inthe forming of the record itself. In the taking of a record the phonograph needle is pressed against the record surface by a force of amount (a) which causes the needle to be normally displaced into the record material by the amount (a). A sound wave incident upon the recorded diaphragm superimposes upon the initial biasing pressure (a) a varying impressed force as indicated by curve 2.

. which represents precession into the record being much lessthan that representing recession. This inequality in the eiiect produced by the two halves of the incident wave is manifest in the wave imprinted upon the record as a distorted wave, as illustrated by curve 3. It will be seen that cach point on curve 3 is reproduced from a corresponding point on curve 2 thru the translation of characteristic 1. Curve 3 may be considered to represent in inverted form the wave traced on a record as seen enlarged in cross section. It illustrates the observed condition that the troughs of the waves, corresponding to the upper halves of curve 3, are flattened out.

Upon reproduction, this distortion is apparent to the ear as a note of the same frequency as the original but differing in timbre or quality. The change in wave shape is evident to the ear as a change in timbre because of the addition of harmonics brought about by the non-linear characteristic 1. Were the original wave 2 of complex wave form, as is the usual musical note which itself has harmonics, then the distortion imposed by curve 1 may so materially modify or modulate the wave by the addition of further harmonies as to materiall depreciate the musical value of the repro uction.

In accordance with the present invention, distortion of this kind is corrected by introducing a counter-distortion having an opposite effect upon the Wave shape. The sum of the two distortions is thus made substantially zero. This correcting distortion is obtained by introducing somewhere into the phonograph system, either in the recording, in the copying of records, or in the reproducing of the sound waves, an element having a forcedisplacement characteristic the opposite of that normally obtaining.

Such a characteristic is illustrated in curve 4 of Fig. 2. The curve is plotted in'electrical terms analogous to the mechanical terms employed above, because the specific means employed in the present invention for obtaining it are electrical. The impressed force is represented, inthe abscissa, as impressed E. M. F. andjh displacement, in the ordinate, as current. It will be noted that the curve 4 is concave, the current varying more rapidly than the first power of the E. M. F. and is the opposite type of characteristic to curve l of Fig. 1 which is convex because the displacement varies at a rate less than the first power of the force. For perfect correction the two characteristics should be as nearly as y in lili

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possible conjugate functions. llf for instance in curve 1 the displacement varies as the square root of the impressed force, then the correcting characteristic, curve l, preferably should be such that the displacement current varies as the square of the impressed force.

lignre 2 demonstrates the correction of distortion of the type arising in Figure l because the distorted output wave 3 of ltigure 1 is shown to be the wave 3 fed into the device having characteristic d, and the resultant output wave 5 is seen to be of substantialtic of lligure 2 is intended to represent over.-

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all correction irrespective of where it is introduced into the phonograph system, whether previous or subsequent to the point in the process at which the major distortion occurs. Furthermore, this correction may be lumped in a single stage of the phonograph process, as in recording, or may be distributed among several stages or devices, as illustrated and explained below.

lli specific means for carrying out the distortion correcting action described above is shown in llig. 3 wherein said means is employed in the reproducing of sounds from a record, i. e., in a phonograph device proper. 'llhe undulations of the record 'l0 actuate the electromagnetic device 11 which is similar to an ordinary polarized telephone receiver. (This device may be of any of several well known types such as a carbon microphone transmitter with polarizing battery or an electrostatic transmitter.) AlElectric currents,

carrying the record distortion, arethereby generated and are transmitted through in-` put transformer 12 to the well known audion or thermionic amplifier tube 13. rllhe cur- .rents are vthus amplified and then passed. to

a second audion tube 15 through a suitable input transformer lll. rllhis tube is adjusted by means of a negative C battery and potentiometer 16, to operate at a point around the lower knee of the characteristic curve.

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obtained. llube'lti is therefore operated as a distorter' or modulator as distinguished .from the purely amplifying tube 13. "lhe 'recorded enter and operate the electromagnetic translating device 19 illustrated as a telephone receiver.- The current waves thus generated in the receiver windings flow through the input transformer 20 and are impressed upon amplifier 21 and in turn lit) upon lthe distorting tube 23. rllhe operation'l of this tube is similar to that of the corresponding tube 15 of llig. 3. The currents emerge from theoutput transformer 25 predistorted and are impressed upon the translating-device 26 and the undulations are thereby impressed upon the record 27. rllhe distortion imparted to the sound waves while in electric form by 23 may be made sufficient to compensate for all the distortion of this type which will be experienced in the subsequent copying upon original record 27 and reproduction in a phonograph. rllhat is, the master record may be not only corrected but also predistorted to compensate for any amplitude distortion which may occur in the following steps of the phonograph process. 'lhis is what l regard as one of the important embodiments of the invention.. lBy thus over-distorting, as it were, during the recording process by an amount corresponding closely to the distortion which, it having been determined in advance by wave analysis will obtain in the reproducing machine, t e record p-redistortion will automatically neutralize the distortion of the phonograph machine. ln this way by a single use in recording of the amplifying and distorting means, volume and quality improvement is made eiective, through the records, to all phonographs without modification of them. 'lhe economic advantage of this method of correcting distortion in ad* vance in the records as compared with that of doin it in each reproducing machine individua ly, is obvious. 'lo obtain the best results it is necessary that all phonograph reproducers be held7 during manufacture, to close limits as to their distortion characteristic.

A further improvement in the quality of reproduction may be obtained by combining means for correcting magnitude non-linearity distortion with means for correcting distortion of the natural ,period type. This combination is illustrated in lf ig. 5 where'the means for correcting the latter type of distortion are indicated as 31. rlhis consists of a shunt across the circuit comprising a seriesiltli lill lift

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lllll llt@ connected inductance capacity and resistance The circuit of Fig. 5 is generall similar to those of Figs. 3 and 4 and nee not be described in detail. The theory of the action of such a selective shunt as'31, in correcting for inequality in the efficiency with which different frequencies of the sound wave spectrum are transmitted, is elucidated in my copending application previously referred to. Such a selective shunt acts to impart to the circuit a frequency-transmission characteristic the .opposite of that normally obtaining.` It reduces the efficiency of the system in a range of frequencies at which the efiiciency is disproportionally high because of resonance effects in diaphragme etc. Fig. 5 therefore illustrates a distortion correctmg means operative for correcting both types of distortion which are due to non-proportionality of reproduction in respect to (a) amplitude variation and (7)) frequency variation.

Fig. 6 illustrates another arrangement for accomplishing the same result. The means for correcting ty e (b) distortion is changed from that of a se ective shunt 3l of Fig. 5 to a selective circuit 39 forming a part of a feed back circuit of say the first vacuum tube stage. The feed back circuit is formed by connecting the output circuit back upon the input circuit, the coupling being between the input coil 36 and the coil of the output selective circuit 39. The selective circuit 39 is tuned to -the frequency of the natural eriod which it is desired to compensate. ontrary to the usual feed-back circuit employed for increasing the effective amplification, the coil of circuit 39 is poled in a mutually opposing manner with respect to input coil 36. The feed-back currents oppose the currents incident-upon the input circuit from the originating source and since the feed-back action is a maximum for frequencies corresponding to the natural period in question, a selective reduction in the effect-ive amplification of tube 37 is effected at said natural period. The tuning may be done in either the output circuit as illustrated or in the input circuit. Furthermore either or both of said circuits may be tuned to a plurality of frequencies corresponding to a plurality of natural periods of the phonograph system.

The invention is not intended to be limited to the correction of distortion arising from force-displacement characteristics of the type of curve 1 0f Fig. 1.

Considering for the moment this curve to represent the overall performance of the system including both recording and reproducing. then it may be assumed to represent the composite or resultant of two curves of different shapes summed together, one having a greater and the other a lesser slope than curve 1. One of these component curves may represent the distortion of the recordin and the other of the reproducing process. f in this case the distortion is corrected entirely at the recording end, then a single device having a. characteristic of Fig. 2, may be employed. If, however, the distortion of the two sta es be corrected individually then the recor er distortion correcting device is designed to have the characteristic necessary for counteracting its distortion, and similarly with the reproducer.

The charzu'teristie which actually obtains in a practical case may differ materially in shape from curve l, may be irregular in shape or may even obey an entirely different law. As an extreme and not representative case for example, a phonograph device may exhibit the type of characteristic shown in curve 4 of Fig. 2. In this case an equalizing characteristic of the type of curve 1, Fig. 1 would be employed. This latter general type of curve may be also obtained from a thermionic tube by using the upper or saturation knee of the familiar potential current characteristic curve, as illustrated by curve 50 of Fig 7. In Fig. 7 the lower portion of curve 50 corresponds to curve 4 of Fig. 2 and is intended to follow substantially a second power law. The upper knee corresponds in a general way with the characteristic of curve l, Fig. 1. Considering now the manner in which characteristic 50 is employed in compensating for the distorting effect of curve 4.-,the audion tube is adjusted by means of its C or biasing battery to a point on the upper knee such as 51 of Fig. 7. The tube then operates to distort or modulate the currents in a manner the opposite of the phonograh device having characteristic 4.

Fig. 8 illustrates vacuum tube means for controlling the shape of the saturation knee of the characteristic 50 of Fig. 7, to the end that the distortion when employing this portion of the characteristic may be readily controlled. The general circuit of Fig. 8 is similar to the preceding ones and need not be described in detail. The first tube is intended as a straight amplifier while the second one is the distorter for which curve 50 obtains. The curvature of the saturation knee is controlled by a variable resistance 52 in the battery supply circuit. By increasing this resistance from some nominal value up to a value comparable to the internal plate resistance of the tube, the up er knee may be greatly sharpened. This e eet of sharpening the saturation knee by the use of a rela-tively high value of resistance in the yplate circuit is not novel with the present4 invention, it being already known in vacuum tube engineering. It is caused by the battery supply circuit resistance taking up a controlling proportion of the plate circuit voltage as the internal plate-filament resistance drops with increase of impressed potential.

A further feature of the present invention resides in the use of these vacuum tube dislll neaaara y l .nu

torters in changing and controlling aty will the timbre ot music rendered :trom a phonograph, irrespective ot the distortion correcting uses previously described.. llhat is, distortion may be intentionally introduced in this way into the music as the phonogra" h is being played, tor` the purpose oi ena. ling personal expression to be imparted to the rendering thereot. 'llhe characteristic employed tor this purpose is preferably one giving distortion rich in harmonics., The saturation lmee ot curve 50, sharpened by such means as are illustrated in Fig. b, is employed in communication engineering i'or the f production oi harmonics and is well adapted tor this purpose in music. ln producing such.

timbre or tone edects the harmonic-producing tube may be intentionally soA overloaded that it operates over its entire characteristic, curve including both lower and upper lrnees. 'llhis obviously may be done also in the correcting ot distortion where vthe characteristics to be edualized are complex..

Another ileature illustrated in lligs. 'l' and n is the use oit a band blter itin the output circuit tor the purpose ot eliminating unde till till

sired 'frequency components. 'llhis iilter has the property oi transmitting a band ot ireeluencies lying within certain limiting treopiencies and oit cutting odi' all frequencies outside ot said band. .The band tilter illustrated is but one ot several possible terms all oi the general type ci blter circuit disclosed in lll. S. Patent blos. 1,227,3ll3 and l,22l,lll to George A. Campbell buch lters may be used to suppress all harmonics above a certain treoluency or may be employed in reducing or eliminating l'undamentals below a. certain trequency or lor suppressing i'requencies both above and below certain limits.. p

lln lhlig. 8 means are provided in the potentiometer 53 ior controlling the amplitude or volume et reproduction, and in the variable capacity oli selective shunt 3l tor controlling the treeluency equaliaing action, and in the variable resistance 5% lor controlling the timbre, all in accordance with the will ot the player or operator. llt will be understood theretore that in accordance with lig. d, the present invention provides means tor controlling and manipulating the amplitude or volume and the frequency composition or tone quality oit a phonograph during either the malrin ot the records or their reproduction in a p onograph instrument..

lErom the above description it will be apparent that one et the important leatures ot the invention is that ol ampli'ltying the sound wave energy., By so doing it is possible not only to otl'set the losses incurred in the translating actions, but to over-compensate the same so that the energy ol2 the reproduced sound is greatly enhanced. llt is particularly advanta cous to employ this amplilying tea ture ot t e invention in the original'record-A ing et the sound waves tor the purpose oit diminishing the scratching sound so common to the every day phonograph. For this ur-I pose the increased energy made availab e in the recording electromotor device such as 26 llt oi Fig. 4l is used toincrease the ratio oi' the sound wave undulations oi the reco-rd to the irregular undulations causing the scratchm llhis may be done by allowing the increased energy to impress undulations in the record out amplitude larger than heretofore. By then operating the reproducing instrument at reduced sensitivity, as by increasing the damping ot its diaphragm, the extraneous scratch will be lost out in comparison with the desired sound. Another way ot similarly employing the increased energy is to increase oi material less sott than is the usual wax.

record. Such harder records may be ein ployed tor the purpose ot obtaining sharper records which improve the reproduction oi the higher -lreqLuency components, or with the object el" obtaining greater homogeneity ot texture and thereilore less irregularity and resulting scratch, or tor purposes oi' tacilitating the reproduction and manufacture ot records.

@bviously many variations are possible in the means employed tor producing the de sired results without departing trom the intent and spirit ot the invention as detined in the claims below. For example, the non linear characteristics illustrated as being obtained by the use ot a vacuum tube may be derived trom any oi a number ot devices. 'lFhey may -lor. instance be realised by the use oit' iron coreV coils,` the permeability ol which cores vary with current strength, or by the use ot' mineral contacts commonly employed as detectors in radio telegraphy, etc.

hlor is the invention to be interpreted as being limited to the use ot non-linear characteristics obtained electrically and applied through an interposed electric system since other means, such as mechanical, may be employed. ller example, distortion correction may be obtained by transll'erring the record wave; such as 3 o'lB llig. 1, onto a second record, applying the wave in reverse manner. lin the hill and dale type ot record this would mean that the valley halves oi the waves of the lirstrecord become the hill halves ol the waves oil the second record. lllurther more, it is obvious that the invention is not lill bil

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limited to the mechanical type of phonograph illustrated but may be applied as well to a sound reproducing system of any type, as to magnetic phonographs similar to the telegraphone Where amplitude Y distortion tends to occur because of the non-linear nature of the characteristic B`H curve of the iron, etc.

Although I have herein described only certain speciiic manners of practicing the invention and illustrated and described only certain specific forms of embodiment thereof, it will be readily understood that man changes and modications may be made therein without departing from the spirit and scope of the invention, it being my invention to claim the same broadly in whatever form its principles may be embodied.

I claim:

l. In a sound recording and reproducing system, the method of correcting amplitude distortion which consists in introducing into said system an amplitude distortion having distorting effect the opposite of said original distortion. v

2. The method of modifying the tone qual- -ity of aplionograph system, which consists in imposing on the Waves passing through the system and amplitude correction of a character predetermined With reference to the distortion in the record.

3. The method of modifying the tone quality of a phonograph system which consists in imposing on the waves passing through the system an amplitude correction of a character predetermined with reference to the distortion in the record by causing changes disproportionately to each other in the successive instantaneous. amplitudes of the Waves.

4. The method according to claim 3, in which the vibrations representing the sound are converted into electrical undulations, and in which the said disproportional changes in amplitude are imposed on said electric undulations by passlng the wave through an element of the electrical system having a non-linear current voltage characteristic.

5. The method of correcting for the combined amplitude and resonance distortion suffered by waves passed through a phonograph system, Which consists in imposing on the waves an amplitude distortion substantially equal and opposite tothe amplitude distortion suffered by the Waves because of said system, and imposing on the Waves a resonance distortion substantially equal and opposite to the resonance distortion suffered by the Waves because of said system.

6. The method of im arting a desired timbre to a musical note o tained from a sound producing system, which consists in impos- 111g on the waves representing the note an amplitude distortion of such predetermined character as to give rise to harmonics of said note.

7. The combination with a phonographic system wherein the sound Waves suffer amplitude distortion, of means subjecting the w aves to substantially equal and opposite amplitude distortion.

8. The combination with a phonographic system wherein the sound waves suier amplitude distortion, of means subjecting the Waves to substantially equal and opposite amplitude distortion, comprising means for converting the waves representing sound into electrical undulations, and means having a y non-linear voltage current characteristic interposed into the path of said undulations.

9. The combination according to claim 8, in Which the said non-linear means comprises an electron tube, and means for causing the same to operate on a curved portion of its voltage current characteristic.

10; The combination according to claim 8, in Which the said non-linear means comprises an electron tube, a polarizing source of electro-motive force, and means for operatively associating the same with the tube.

11. The combination according to claim 8, in which the said non-linear means comprises an electron tube of the three-element type and a polarizing source of electro-motive force in which the negativey ole of the source is cony nected to the grid e ement of the tube so as to cause said tube to operate at about the lower knee of its voltage current characteristic curve.

12. The method of obtaining a record substantially free from amplitude distortion in the Waves recorded thereon, which consists in imposing on the Waves before being engravcd on the record an amplitude distortion substantially equal and opposite to the distortion suliered by the Waves when being engraved in the record material.

13. In a musical instrument or system, the combination With means for producing musical notes, of means for changing the timbre of the notes by impressing on the Waves representing the notes a non-linear distortion of predetermined character to give rise to harmonics of the notes.

14. In a musical instrument or system, the combination with means for producing musi-- cal notes, of means for converting the sound Waves into electrical undulations, and means for modifying the electrical undulations to change the timbre of the notes as they are finally emitted by the instrument, comprising a non-linear translating device for impressing on the undulations a non-linear distortion of predetermined character to give riseto harmonics of the notes.

15. In a musical instrument or system, the combination with means for producing musical notes, of means for changing the timbre of the notes, comprising a device for impressing a non-linear distortion of predetermined character on the waves representing innata the notes to give rise to harmonics ot the notes, and means tor ltering out undesired frequencies.

16. ln a musical instrument or system, the combination with means for producing musical notes, of means tor impressing a nonlinear distortion on the Waves representing the notes, together With means for compensating for resonant distortion by the system.

17. ln a system of sound recordation in which by virtue of record limitations ory lor other reasons the Waves ot greater amplitude have been reduced in comparison With'vvaves ot lesser amplitude, the method ol expanding the amplitude range of the reproduced sound, which consists in converting the reproduced vvaves into electric current Waves, so modulating said current Waves as to enhance the amplitude ot the Waves ot greater amplitude in comparison with those of lesser amplitude, and finally reproducing as sound the currents so modified..

18. rl"he method ot recording and reproducing sound, which consists in. (1) forming a record which is distorted by virtue of the maximum undulations being limited in value to a substantially greater extent than the weaker undulations, and then (2) reproducing said record With an amplitude distortion the opposite ot that carried by the record.

19. 'lhe sound producing machine, comprising means for generating electric Waves, means for developing harmonics ot said Waves, means tor reproducing said Waves as sound Waves, and filtering means tor restricting the reproduced `Waves to predetermined trequencies.

20. lin a musical instrument, the combination ot a source voil current corresponding to sound vibrations, vacuum tube means for controlling at vvill the generation ot desired harmonics and in turn the timbre bil the reproduced sound, means tor controlling the volume thereof., and means for translating the current into sound Waves.

21. 'llhe method ot imparting a desired timbre to a musical note obtained trom a sound producing system, which consists in imposing on the Waves representing the note an amplitude distortion of such predetermined character as to give rise to harmonics of said note, and then filtering the desired trom the undesired components.

22. lln an electrical sound producing instrument, means to produce alternating currents of the acoustic range ot frequencies, and electric circuit means tor reproducing such currents as sound Waves, comprising means in said circuit tor controlling the volume ot the sound vvaves and for independently giving them a desired timbre.

23. lin a phonographic system, the method of transferring sound Waves or their equivalent, which consists in amplifying them, generating harmonics ot said Waves by subjecting them to the action ot a non-linear translating device, and'linally selecting by electric tilter circuits the frequency range to be reproduced.

2t. ln a phonograph system, means lor converting sound vvaves into electrical Waves, means 'for distorting said electrical Waves to offset or neutralize resonant distortion ot the system, means tor ed'ecting a non-linear distortion ot the Waves to give rise to harmonics', means tor re-converting the electrical Waves into sound Waves and reproducing them as sound, and means tor controlling the volume of the reproduced sound..

ln testimony whereof, l have signed my name to this specification this 16th day ot' December, 191

LlJUllDt ESPENSGHMED. 

